The Art of Selection
Oct 1, 2007 12:00 PM, By Michael Eby, Editor-in-Chief
On the surface, incorporating the concepts of selective coordination in the design of power distribution systems seems like a “no-brainer.” Why wouldn't you want to design a system that localizes an overcurrent condition and restricts the outage to the affected circuit or piece of equipment? We all know how much we've come to rely on 24/7 power in our homes and, more importantly, in our places of business. The entire loss of power in a high-rise condo, large office building, industrial plant, or hospital building is simply unacceptable in today's fast-paced “always-on” world.
But should the concepts of selective coordination be applied to emergency and standby systems too? Are there valid safety risks at play that should require us to incorporate these concepts into our electrical designs? More importantly, should these perceived safety risks merit specific Code requirements as set forth in the NEC? According to the 2008 NEC, the answer remains yes. However, there are those in the industry who disagree — and continue efforts to try and reverse the decision to make this a national mandate.
As an engineer, I tend to agree with their arguments, and wonder why the NEC must mandate the issue of selective coordination for emergency and standby systems. Incorporating these concepts into the design of these types of power systems can be very difficult to achieve, not to mention very costly to implement. Where's the background data that shows this is an issue that affects public safety?
As far as I know, NFPA 110, “Standard for Emergency and Standby Power Systems,” and IEEE Standard 242, “Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems,” both recognize selective coordination as a design consideration, not as a requirement. So why not leave this decision in the hands of the professional engineer until proven data clearly shows this to be an issue of public safety?
To learn more about this controversial issue, turn to the cover story on page 42 and take a look at what Staff Writer Beck Ireland dug up on this topic when speaking with some savvy engineers and Code specialists who are trying to work these requirements into their new designs. The information she presents on how some states and local jurisdictions are allowing exceptions to these requirements is quite interesting.
Do you agree with these exceptions, or do you feel the NEC should continue to mandate selective coordination for emergency and standby systems? I'm very interested in your feedback on this important topic. Please shoot me a quick e-mail at mike.eby@penton.com, and tell me what you think.
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